Forces and Motions

ForcesWhat is a Force? Anything that changes

the state of rest or motion

of an object It’s what causes

ACCELERATION

has magnitude and direction Therefore force is a vector

Force cont.A force can cause a

resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

Units = Newtons (N)

Forces can be COMBINED:Net Force: the combination of all of the forces acting

on the object

Balanced Forces: When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion.

Warning: Doesn’t always mean

the object is at rest;

Example: Tug of war,

Pushing piano, car

traveling at constant

velocity

Unbalanced Forces Results when net force acting on an

object is NOT equal to zero When an unbalanced force acts on an object,

the object accelerates in the direction of the net force

Fig 14, p.332

http://www.darvill.clara.net/enforcemot/friction.htm

*(moves in direction of net force)

Force

A push or pull on an object Units: Newton (N) = 1 kg x m/s2

Net Force (Fnet) : sum of all of the forces that are acting on an object

Balanced Forces: Fnet = 0 Object is at rest or moving with constant velocity

Unbalanced Forces: Fnet is not equal to 0, Object is accelerating or decelerating

Types of Forces Applied – contact force in the direction the

object is moving Tension- caused by a rope, cable, etc.,

directed away from the object Friction- opposes the motion of objects,

must have contact Normal- caused by a surface Force of gravity- force at a distance, caused

by attraction between two objects

Two Types of Friction Static

Friction of an object at rest Greater than kinetic friction (harder to get an

object moving than to keep it moving) Friction is greatest when surfaces are rough

ex. the force that is keeping this block from

sliding downhill Kinetic

Friction of a moving object

Types of Kinetic Friction

Sliding Two surfaces moving past each other

Rolling Movement of a circular object on a surface Ex. Cart on wheels

Fluid Friction Movement of an object through a liquid or gas Ex. Air resistance

FRICTION

STATICSLIDING ROLLING FLUID

The friction between surfaces that are

stationary

force that exists when objects slide

past each other

force that exists when a round object

rolls over a flat surface (usually less than sliding friction)

force that exists when an object

moves through a fluid (air, water)

a book sitting on a table

-hockey puck on ice-child going down a

slide-a sled down hill

-a roller blade on a sidewalk

-bowling ball on bowling alley

-a car driving down the road

-swimmer swimming through pool

is defined as is defined as is defined as is defined as

& an example is & an example is & an example is & an example is

Can you think of a situation in which you would want to increase friction?

How can we decrease friction?Watch this demo and see…

http://www.scottso.net/examples_encfri.htm

Gravity is a force: natural phenomenon in which objects that have mass are

attracted to one another

Gravity is an attractive force pulls objects together

Earth’s gravity acts downward toward the center of the Earth.

There is an upward force that balances gravity AIR RESISTANCE

Falling Objects have two forces acting on them:

Gravity causes objects to accelerate downward air resistance acts in the direction opposite to

the motion & reduces acceleration.

Free Fall: the motion of a body when only the force of gravity is acting on the body

Free fall acceleration of an object is directed toward the center of Earth

Because free fall acceleration results from gravity, its symbol is g

Acceleration due to gravity on Earth = 9.8 m/s2

Formula for objects in free fall: d = ½ gt2

In a vacuum, two objects would accelerate at the same rate because both are in free fall (Fig 8, p.354)

Question: What other force is not present in vacuum that would affect acceleration?

Answer = air resistance

Drawing Free Body Diagrams

Identify the types of forces acting on the object Use a dot or a box to represent the object Use arrows to represent the direction and size

of the force -change the size of the arrow if more force is

being applied in one direction versus the other Label the arrows with the type of force

Free Body Diagrams

How will the object move in the diagram to the right?

Answer: Object may be at rest and not move or it could be moving at a constant velocity since all the forces are balanced and the net force is zero

Free body diagram practice

Free Body diagram practice

II.) Newton’s First Law of MotionA) Historical Development

1) Aristotle (384 BC- 322 BC): Incorrectly proposed that force is required to keep an object moving

2) Galileo (1564 – 1642): Concluded that moving objects not subjected to friction or other force, would continue to move indefinitely; Disproved Aristotle

3) Newton (1643 – 1727): Defined mass and force; Introduced 3 Laws of Motion

C) Newton’s 1st Law of Motion 1) According to Newton’s 1st Law, the state of

motion of an object does not change as long as the net force is zero.

a) Basically saying that an object at rest will stay at rest unless a force acts on it

1) Example: Soccer ball will remain (at rest) on the grass unless a force is

acted on it

2) Sometimes called the “Law of Inertia”

a) Inertia: the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object

1) Car crash: You continue

forward because of inertia

“Science and the Consumer”

p.348

How is inertia related to mass?P 347 Mass is a measure of inertia.

Who would you rather be tackled by…a toddler or a defensive lineman?

What is easier to move? An empty garbage can or a garbage can full of lead? Why? The empty garbage can has less mass= less inertia=

less resistance to being accelerated.